The present invention relates to the use of crystalline organic molecules in non-linear optics. More particularly, this invention relates to the use of a chiral molecule having harmonic generating substituent groups.
Non-linear optics is means to cover those fields of optics in which one frequency of radiation is converted to another, and to electrooptical modulation of radiation, in which only one characteristic of the radiation is modified. Frequency conversion is obtained from the mixture of two radiations of different frequencies. The conversion is equal to the sum or the difference of frequencies of the incident radiations. Electrooptical modulation can be obtained by the application of an electrical field to a transparent crystal traversed by the radiation.
It is known from the prior art that inorganic crystals can be used to obtain frequency doubling. The most widely used frequency doubling crystals are based upon potassium diphosphate KDP or lithium niobate. KDP suffers from inadequate frequency conversion efficiency, thereby requiring the use of crystals of considerable thicknesses. Since the heat dissipation and thermal failure properties of the crystal are a function of its thickness, a shorter crystal is desirable. Lithium niobate cannot be grown in large sizes, and is not suitable for generating blue or ultraviolet light.
There is a continuing research effort to develop new nonlinear optical organic systems. Organic systems offer molecules with large delocalized .pi.-electron systems which exhibit non-linear optical response, typically much greater than the response obtained from an inorganic molecule. Further, it is known that organic molecules can be modified to optimize other desirable physical properties to achieve high mechanical and thermooxidative stability, while preserving the desired non-linear optical response, useful for frequency conversion.
Examples of some organic compounds which have been identified are 13,13 --diamino--14,14 - dicyanodiphenoquinodimethane (U.S. Pat. No. 4,707,303), paranitroaniline derivatives (U.S. Pat. No. 4,622,409), and 3 and/or 5 --methyl--4 --nitropyridine--1 --oxide (U.S. Pat. No. 4,376,899.)
What is known generally about the frequency doubling crystals is that its molecule had to have a noncentrosymmetric crystal structure and have sufficient birefringence for phase matching. Non-centrosymmetric species are those which have no center of symmetry on either the molecular or crystalline unit cell level. Despite this generalization, there was no general method for identifying crystals with these properties which were also available for doubling the frequency into the blue or ultraviolet spectrum. Thus, finding new frequency conversion crystals was an empiric, hit or miss process.
The present invention establishes a way of constructing the candidate molecule to ensure these known required properties with high probability by disclosing a third, chemical criterion that is required for these crystals to have useful nonlinearities and optical transmission properties.
Specifically, this invention adds the requirement that frequency doubling crystals contain a "harmonic generating unit" with the following properties. The molecule, or molecular unit, must be intrinsically noncentrosymmetric itself in order to be a "harmonic generating unit." The unit must have relatively low energy electronic excitations which simultaneously possess both strong 1 and 2 photon dipole transition moments with the electronic ground state. The molecular unit must be anisotropic, e.g., planar or "one dimensional" in geometry. The harmonic generating unit must be transparent to visible and ultraviolet wavelengths. Optionally, the unit is formed from ionic materials so that crystals of the compound can be formed as salts with readily available anions and cations.
Crystals formed from chiral molecules containing such harmonic generating units are of necessity, noncentrosymmetric and have high probability of being very birefringent. Therefore, the first two requirements for harmonic generation are automatically met. These same properties also favor substantially nonlinear crystals which improve efficiency. The transparency condition on the harmonic generating units guarantees that crystals formed will be transparent to the doubled light.
The present invention provides frequency conversion crystals which are formed from organic chiral molecules having a harmonic generating unit which has optimum properties for doubling or mixing specific wavelengths and which lends itself to some degree of tailoring a crystal's phase matching properties by using certain chemical substitutions to change dispersion of the refractive index in the near infrared or near ultraviolet wavelengths. Examples of this chemical substitution are deuteration, halogenation and the substitution of cations and anions.